During development, gustatory innervation is a tightly regulated process in which specific numbers of primary taste afferents project to discrete regions of the oral cavity. The present research is designed to investigate gustatory innervation in support of two long terms goals: 1) to determine the cellular and molecular mechanisms that allow gustatory neurons to innervate the correct target (taste buds) with a specific mount of innervation, and 2) to determine if and/or how these connections are important for central gustatory development or taste function. The proposed project focuses on the role of neurotrophins in regulating gustatory neuron number and peripheral and central targeting during embryonic development. It is known that the number of gustatory ganglion neurons that will exist at birth to innervate taste buds on the tongue and palate is somehow determined by brain-derived neurotrophic factor (BDNI0 and neurotrophin-4 (NT4). However, the mechanism(s) by which BDNF and NT4 regulate innervation are unknown, and could involve any number of different developmental processes during numerous different embryonic stages. The proposed studies combine in situ hybridization, cell counting, tract tracing, immunohistochemistry, and SEM in gene knockout mice to: 1) determine when and where the embryonic taste system is exposed to BDNF and NT4, 2) determine when BDNF and NT4 regulate gustatory neuron number and whether this regulation is accomplished by reducing proliferation or by increasing cell death, 3) determine whether BDNF and/or NT4 is required for initial target invasion, and 4) determine if BDNF and/or NT4 regulate the central distribution of tongue and palatal sensory afferents. Together, these studies test the hypotheses that: 1) NT4 is expressed along gustatory fiber projection pathways and regulates gustatory neuron survival before initial innervation of peripheral and central targets, and 2) BDNF is expressed in gustatory epithelia and regulates neuron survival after initial peripheral and central target innervation. Because these experiments examine how neurotrophins regulate the amount and location of sensory innervation, this project has important implications for the potential therapeutic use of these powerful signaling molecules in controlling neural regeneration following injury.